Klaus Frühmann

Structure–function relationships of four compression wood types: micromechanical properties at the tissue and fibre level

The mechanisms behind compressive stress generation in gymnosperms are not yet fully understood. Investigating the structure–function relationships at the tissue and cell level, however, can provide new insights. Severe compression wood of all species lacks a S3 layer, has a high microfibril angle in the S2 layer and a high lignin content. Additionally, special features like helical cavities or spiral thickenings appear, which are not well understood in terms of their mechanical relevance, but need to be examined with regard to evolutionary trends in compression wood development. Thin compression wood foils and isolated tracheids of four gymnosperm species [Ginkgo biloba L., Taxus baccata L., Juniperus virginiana L., Picea abies (L.) Karst.] were investigated. The tracheids were isolated mechanically by peeling them out of the solid wood using fine tweezers. In contrast to chemical macerations, the cell wall components remained in their original condition. Tensile properties of tissue foils and tracheids were measured in a microtensile apparatus under wet conditions. Our results clearly show an evolutionary trend to a much more flexible compression wood. An interpretation with respect to compressive stress generation is discussed.

Properties of chemically and mechanically isolated fibres of spruce (Picea abies [L.] Karst.). Part 3: Mechanical characterisation

Abstract Microtensile investigations were carried out on chemically and mechanically isolated fibres that had similar dimensions before isolation. The properties calculated for the fibres were nearly identical for the two isolation methods. However, cell wall cross-sections of chemically isolated fibres shrank much more transversely than those of the mechanically isolated fibres.

Properties of chemically and mechanically isolated fibres of spruce (Picea abies [L.] Karst.). Part 2: Twisting phenomena

Abstract The twisting behaviour of chemically and mechanically isolated fibres of spruce (Picea abies[L.] Karst.) was examined. Mechanical isolation was carried out using very fine tweezers to obtain fibres with an unmodified cell wall assembly. Chemical isolation was achieved using hydrogen peroxide and glacial acetic acid, leading to partial degradation of lignin and hemicelluloses. Besides normal adult wood, compression wood and opposite wood fibres were investigated. Fibre twisting while drying increased with higher microfibril angles in the S2 layer, and was significantly less pronounced for mechanically isolated compared to chemically macerated fibres. A simple model is introduced that takes into account the interdependency between lateral cell-wall shrinkage and the microfibril angle in the S2 cell wall.

Fracture characteristics of wood under mode I, mode II and mode III loading

Abstract Wood is a highly optimized cellular orthotropic material. Its structural properties have major influence on the fracture mechanics behaviour of the material. The aim of the presented study is the evaluation and analysis of differences in the fracture process of wood subjected to fundamental fracture modes. The results of fracture mechanics experiments for mode I, mode II and mode III in two crack propagation systems were evaluated. The fracture process is discussed by means of the load-displacement diagrams recorded under stable crack propagation and the calculation of the specific fracture energy. It was found that the typical specific fracture energy is much higher for the mode II and mode III cases than for mode I loading. The differences for wood are explained by different energy-dissipating processes such as the development of larger damage zones for mode II and mode III loading than for mode I loading. The influence of the chosen orientation and its relation to structural parameters of the material are also discussed. The findings are supported by microscopic images of typical fracture surfaces.

Damage and fracture mechanisms during mode I and III loading of wood

Summary Tests under mode I and mode III loading were performed on side grooved Compact-Tension specimens of larch and beech under steady state crack propagation to study the damage and fracture behaviour and the influence of two fibre orientations. From the complete load-displacement diagram, all important damage and fracture mechanical values (stiffness/compliance, microstructural damage, crack initiation energy, specific fracture energy, etc.) have been determined. Crack initiation energy and specific fracture energy are approximately ten times higher for mode III loading than for mode I loading in both wood species. Crack initiation occurs in mode III under external mode III loading, crack propagation, however, takes place under mode I, owing to crack surface interference. The influence of fibre orientation on the (fracture) mechanical properties of beech and larch is different. This difference may be explained mainly by the high number of rays in beech.

In situ longitudinal tensile tests of pine wood in an environmental Scanning electron microscope

Summary To study wood fracture on its cellular level, small tensile specimens of pine (Pinus sylvestris [L.]) were fractured in situ in tension inside the chamber of an ESEM (Environmental Scanning Electron Microscope). Fractured surfaces of macroscopic tensile test specimens were also studied with an ESEM. The same kind of fracture phenomena were observed in both small and large specimens. The in situ tests proved to be reproducible and the results revealed typical fracture propagation0 directions and order in softwood under longitudinal tension. The gradual change of material properties of wood in the radial direction was found to strongly influence the fracture process.

Fracture behaviour of Laminated Veneer Lumber under Mode I and III loading

Abstract Mode I and Mode III loading experiments were performed on side grooved CT specimens of two types of Laminated Veneer Lumber (LVL). Steady state crack propagation was maintained in order to detect complete load displacement diagrams. Fracture behaviour and influence of fiber orientation were studied and all important fracture mechanical values (stiffness/compliance, microstructural damage, crack initiation energy, specific fracture energy etc) were determined. Much higher crack initiation energies and specific fracture energies resulted in mode III loading than in mode I loading for both material types. Under external mode III loading, crack initiation occurs in mode III and crack propagation however takes place under mode I owing to crack surface interference. The influence of fiber orientation on fracture mechanical properties of LVL was discussed.